(1) Field of the Invention
The present invention relates to a system and method for controlling automatically a vehicle speed to a desired cruise speed in which an opening angle of a throttle valve is controlled so that the vehicle speed coincides with a target vehicle speed at which the vehicle speed cruises.
(2) Background of the Art
A previously proposed various automatic vehicle speed controlling systems will be described below.
In the previously proposed automatic cruise speed controlling system, a target vehicle speed setting mechanism is provided through which the vehicle speed is set to a target vehicle speed at which a vehicle in which the system for automatically controlling the vehicle speed to a cruise speed is mounted cruises.
The current vehicle speed is detected by means of a vehicle speed detector. A first subtracter is provided which receives the target vehicle speed and current vehicle speed and subtracts the current vehicle speed from the target vehicle speed. A difference result between the current vehicle speed and target vehicle speed is supplied to a first multiplier. The first multiplier multiplies the difference result by a coefficient and supplies the multiplied result to a drive circuit via an adder. The drive circuit drives a throttle actuator for actuating an engine throttle valve. The throttle actuator actuates the throttle valve to open or close the throttle valve via an accelerator link mechanism constituted by a wire.
Consequently, the opening angle of the throttle valve is controlled so that the detected vehicle speed coincides with the target vehicle speed set through the target vehicle speed setting mechanism.
It is noted that the throttle valve is also controlled separately by means of a depression force exerted on an accelerator pedal.
In addition, the target vehicle speed value of the vehicle speed setting mechanism is supplied to a target opening angle calculator in which a table representing a conversion characteristic to be described below is prepared.
The target opening angle calculator derives a target opening angle of the throttle valve through which the vehicle can maintain the set target vehicle speed using the conversion characteristic and supplies the derived target opening angle to a subtracter.
The subtracter receives an actual opening angle of the throttle valve from an opening angle sensor and the target opening angle and subtracts the target opening angle from the actual opening angle.
The difference between the target and actual opening angles is supplied to a second multiplier in which the opening angle difference result is multiplied by a coefficient.
The multiplied value of the opening angle difference in the second multiplier is supplied to the above-described adder to add it to the other multiplied value in the above-described first multiplier.
Hence, the opening angle of the throttle valve is also controlled so that the detected opening angle coincides with the target opening angle corresponding to the set target vehicle speed.
In this way, the throttle valve is opened or closed in a direction toward which the vehicle speed difference and opening angle difference are both eliminated.
The conversion characteristic is linearly formed and, hence, a linear conversion of the set target vehicle speed is carried out by means of the target opening angle calculator in accordance with an equation expressed as follows. .theta..sub.s =K.times.V.sub.2, wherein .theta..sub.s denotes the target opening angle of the throttle valve, K denotes the coefficient and its value denotes a gradient of the linear conversion characteristic and is set to, e.g., 11/80, and V.sub.s denotes the target vehicle speed.
However, it is noted that the relationship between the target vehicle speed at which the vehicle cruises and the opening angle of the throttle valve through which the vehicle can maintain the target vehicle speed is actually non-linear such that in a high vehicle speed region, the throttle valve opening angle is curved toward a lower direction with respect to the vehicle speed and, in a low speed region, the throttle valve opening angle is curved toward an upper direction. Therefore, the actual vehicle speed becomes above the set target vehicle speed in the high speed region and, on the contrary, the actual vehicle speed becomes lower than the set target vehicle speed in the low speed region.
Consequently, a response characteristic of control is reduced and fluctuations in the detected vehicle speed appear.